This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Unky Dan, johngoble1995, Tesseract, REКT(MLg_Wasted_mLG)REКT, TCIAL, [SRP]☬✙DogSenpai✙☬, atomicwater, Shikamaru, ♤Isaac Clarke, ☢ bouboo ☢, 6F5jedi, Urumasi, Qeqqec, MrLiamTheGamer, |DG| SaintlyDemonic, Mraweshumsauce, PoStMaN, Iron Lich, Mrjas26, Brist | KICKBACK.COM, Patrick, Rextox, martins, tanksinatra, ⎛⎝ƊαƑσχχ⎠⎞, Meatwad(new years), TurnIp, The lonely puma, DarkFox, AtomeDix, Divalent, Bobbylol, VeXan, ~ Segeco